lib: banner purge + WHY-only comment sweep (rule 8)

Every // ---- section banner dies (132 -> 0): names carry the WHAT.
Narration deleted (filename restatements, run-with lines, what-the-
next-line-does); every ref/hare cite, task cite, divergence, ABI/
layout contract, and ownership qualifier kept (borrowed-view lines
restored where the sweep over-cut). Comment-only proven: all 442
walk-workdir .s and 32 import-probe .s byte-identical before/after;
libbyteid 56-roster all-ID.

lib/hash/siphash/siphash.ww

@@ -1,5 +1,3 @@
-// hash/siphash — SipHash-2-4 keyed hash, buffer-based.
-//
 // Mirrors Hare's hash::siphash for the one-shot path: take a 16-byte
 // key and a buffer, return the 64-bit hash. Hare ships a streaming
 // io::stream-backed type; ww's subset doesn't, matching the rest of